Measure the dimensions of your bat. Mark the height of the bat as A and the circumference of your bat as B. Mark the length of the bat's handle as C. Most baseball bats are between 34 and 42 inches long and between 2 and 2.75 inches in circumference.
Draw a pattern on your fabric using fabric chalk. The pattern should be about 6 inches taller than A and about 1 inch wider than the bat's circumference. You may want to make the pattern a bit thinner on the end near the bat's handle. Mark C on your pattern and subtract about a half-inch from the width for the entire length of the handle. Draw another identical copy of this pattern.
Cut into the pattern of the fabric.
Turn the fabric inside out and use your sewing machine to sew one side of the fabric together. Turn the fabric right side out and wrap the fabric around your bat to test its size. If you feel your bat has too much room inside the warmer, cut one inch off on the side of the fabric and repeat the test.
Sew the zipper onto the bat warmer
Test your new warmer. Make sure your bat can't fall out of the warmer. If too much unsealed space remains, unzip the bat warmer and use your sewing machine to seal further.